The use of various types of fibres in the production of concrete, to provide additional tensile strength and reinforce against impact damage and crack propagation, has been known and practiced for a long time. It is also known that while conventional reinforcement and coarser fibres can reduce the larger visible cracking which tends to occur in concrete, only very fine fibres are really effective in combating the development of smaller cracks. However, the fibres which are generally used in concrete, for example synthetic fibres of materials such as polypropylene, are relatively coarse, due to the fact that it is difficult to achieve a satisfactory dispersion in concrete of very fine fibres, and particularly fibres with high aspect ratios, using conventional mixing procedures and equipment. In fact, the uniform dispersion of even relatively coarse fibres in concrete can also be difficult.
It is common for such fibres to be produced as an integral fibrillated tape, and to rely on extended mixing to break down the fibrillation and to disperse the individual filaments, which are still relatively coarse, within the concrete. This system may not always be reliable, and the fibrillated tape is not always broken down into the desired individual filaments, especially since the degree of extended mixing required is in practice frequently not achieved. Even when effectively separated, the fibres may still be too coarse to achieve maximum effectiveness for crack inhibition, particularly against micro-cracking.
Concrete is prone to self-induced cracking and, as it is a brittle material, these cracks propagate readily under relatively low stresses. Concrete fails in tension by progressive crack development rather than the more usual failure mode of engineering materials.
It is generally assumed that the discrepancy between concrete's actual and theoretical strength can be explained by the presence of flaws (Neville, A.M., Properties of Concrete, 1981). Thus, concrete does not crack because it is weak in tension, but rather it is weak in tension because it already contains cracks. These cracks and flaws vary in size, so that scale is very important when dealing with fracture mechanics, in that the actual strength of the whole is a matter of statistical probability which is dependent upon the crack distribution within the material. The effective strength of the concrete can therefore be increased, and failure, i.e. the development of large-scale cracks or fractures, can be prevented, by inhibiting the development and propagation of cracks.
Self-induced, non-structural cracks occur in large masses of ready-mixed concrete due to small cracks which form early, and these are subsequently propagated by stresses induced by changes in the dimensions of such relatively large structures. Pavement concrete units are typically about 3 m by 10 m by 200 mm thick; small cracks in such concrete can readily propagate, producing a weak link which results in subsequent fracture. This clearly visible cracking is often the only form of cracking which is perceived as being of importance, but it is a direct result of much smaller and probably essentially invisible earlier crack development.
EP-A-0 235 577 discloses agglomerates of fibres having improved dispersability in viscous organic or inorganic matrices, e.g. cement-based matrices, comprising acrylic staple fibres, each fibre having a diameter of less than 50 .mu.m and length of more than 3 mm, the fibres being bonded to each other by a cohesion-conferring agent which is dissolved in, swells or melts in the matrix to be reinforced. The cohesion-conferring agent, e.g. polyvinyl alcohol, is applied in an amount of 1-30% by weight of the fibre. The fibres preferably have a high elastic modulus.
EP-A-0 225 404 discloses a method of manufacturing a fibre-reinforced moulded cement body, which comprises dispersing strands comprising a plurality of fibres into an unhardened cement material and thereafter hardening the material, at least some of the strands being impregnated with a binder so that the fibres of the strands are weakly bound to one another and so that, when the strands are dispersed in the cement material, the fibres are released from one another. The binder, e.g. an epoxy resin, is used in an amount such that the ratio of the strands to the binder is from 5:5 to 9:1 by volume.
Previous applications of fibre in concrete have been directed towards conventional reinforcement, where sufficient fibres of high elastic modulus are used to bear the tensile stresses. Although this is possible in high cement content materials, this approach may not work effectively with more conventional concretes, even with steel fibres having excellent mechanical characteristics. This can be attributed to the following:
a. The volume of fibre required can be too great to be accommodated in the mortar phase of concrete. PA1 b. The benefit of the fibres may be achieved after the matrix has failed, and can thus in such cases be described as simply being progressive failure rather than usable strength. PA1 c. The cost and difficulties in use do not always justify the application. PA1 d. The three-dimensional orientation of the fibres in premix use and the use of the fibres throughout the material often comprise an inefficient use of reinforcement. PA1 adding to a concrete, mortar or cement mix to which water has been added less than 1% by weight, based on the cementitious materials, of synthetic fibre bundles comprising 10-10,000 filaments per bundle, the filaments comprising a polyolefin such as polypropylene or polyethylene, a polyolefin derivative, a polyester or a mixture of the foregoing and having a length of 1 to 30 mm, a mean transverse dimension of 5 to 30 .mu.m and an aspect ratio of 100 to 1000, the filaments in each bundle being held together by a wetting agent, the wetting agent providing the individual filaments with a surface tension which allows them to become substantially homogeneously dispersed in the mix with conventional mixing in conventional concrete mixing equipment, PA1 mixing the resulting mix for a period of at least about 20 seconds to obtain a concrete, mortar or paste mix in which the individual filaments are substantially homogeneously distributed, and PA1 casting the concrete, mortar or paste mix in a desired configuration, optionally with incorporation, during the casting, of additional bodies such as reinforcement. PA1 melting the fibre raw material(s) to obtain a melt, PA1 spinning the melt into spun bundles of filaments, PA1 stretching the bundles of filaments, PA1 drying and fixing the bundles of filaments, such that the stretched filaments after fixing have a mean transverse dimension of 5 to 30 .mu.m, PA1 treating the bundles of filaments with a wetting agent so as to hold the filaments of each bundle together and to provide the filaments with a surface tension which allows them to become substantially homogeneously dispersed in a concrete, mortar or paste with conventional mixing in conventional concrete mixing equipment, and PA1 cutting the bundles of filaments to a length of 1 to 30 mm, such that the individual filaments have an aspect ratio of 100 to 1000. PA1 more homogeneous and consistent concrete, with more uniform and reliable characteristics, PA1 easier pumping, placing and finishing, and the prevention of sedimentation and excessive bleeding, PA1 a reduced tendency for the formation of plastic settlement cracks, as a result of the reduction in sedimentation, and PA1 benefits when placing the concrete on slopes, as the material has less tendency to continue movement, which otherwise results in increased tendency to cracking.
It has become increasingly evident that the most important commercial contribution of the fibres is to improve the characteristics of the concrete itself, rather than to act independently as a reinforcement.
Reinforcement is, however, easy to measure, and although the other benefits, i.e. the strengthening of the concrete itself, may be recognized as being of significance, the difficulty in measuring and quantifying them has been a factor in inhibiting this application of fibre in concrete.